Phase firing circuits

ABSTRACT

A control circuit for switching single or polyphase electrical supplies, wherein actuation of a single pole switch energises transformer means which control thyristor means adapted to supply electrical load means from a source of alternating current, the electrical load means being electrically connected to said alternating current as the phase voltage is at or near zero volts, in polyphase systems the transformer means controlling thyristor in preceding or succeeding phases, and each phase having a transformer means and thyristor means.

United States Patent [191 Hawker et a1.

PHASE FIRING CIRCUITS Inventors: Collin John Hawker, Broadmayne,

near Dorchester; Wilfred Roy Curle, Martock; Harold David Read, EastCoker, all of England wesliraaisii'aan LiinitdiVeTiv iij' Somerset,England Filed: June 19, 1972 Appl. No.: 263,952

Assignee:

Foreign Application Priority Data July 5, 1971 Great Britain 31,479/71US. Cl. 317/33 SC, 307/133, 307/252 UA Int. Cl. H02h 7/22 Field ofSearch 317/11 A, 33 SC;

307/133, 252 UA; 323/2 S References Cited UNITED STATES PATENTS 11/1966Naber 307/133 SCR1 SCR5

N scR [451 Aug. 14, was

3,373,290 3/1968 Baker 307/133 3,577,177 5/1971 Wilson 323/2 S PrimaryExaminer-James D. Trammell Att0rney-Roberts B. Larson, Ross F. Hunt, Jr.et al.

sranc'r 15 Claims, 10 Drawing Fi gui-es fi g 0R2 D5 T2 D2 minnows 14 umsum 5 or 6 m nnows 14 ms 3. 753; 044

saw 6 0F 6 FIG.6

1 PHASE FIRING crncurrs This invention relates to a method ofcontrolling single and polyphase electrical supplies applied to externalloads, and more specifically to modulation of electrical power to saidexternal loads. Hitherto, switching has been accomplished using relaycontacts in each phase, wherein suppression of R/F interference has beena major problem, or by complicated electronic circuitry in an endeavourto reduce RIF interference in the switching operation. This invention,however, provides a switching means wherein the initial switching isonly in one phase, the circuitry is simple, and R/F interferenceminimal, the invention being suitable for either single or polyphasesupplies.

According to the invention we provide a control circuit for switchingsingle and polyphase electrical supplies wherein the excitation offiring means controls thyristor means adapted to supply electrical loadmeans from a source of alternating current, said electrical load meansbeing electrically connected to said electrical supplies at or near zerovolts, and wherein said single and polyphase electrical supplies areswitched by single pole switching means.

The invention will now be described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 discloses a basic circuit wherein a three-phase supply to athree-phase electrical load is controlled,

FIG. 2 discloses the phase relationship of phase supply voltages andtransformer output voltages,

FIG. 3 discloses the phase relationship at the time of switching on andswitching off the circuit,

FIG. 4 discloses a block diagram of a practical polyphase application ofthe invention,

FIG. 5 discloses a basic circuit wherein a single phase supply to anelectrical load is controlled, and

FIG. 6 discloses the following waveforms of the cir cuit of FIG. 5:

6A T primary waveform 68 T, secondary waveform 6C SCR: gate waveform 6DSCR, gate waveform For ease of understanding a polyphase system, we aredescribing a three-phase system in which the following three assumptionsare made:

Mains sequence is R (Red) Y (Yellow) B (Blue) Phase Retard firing ofS.C.R.s.

Switch S, Closed at Y phase 0 positive going.

In FIGS. 2 and 3, R,, Y, and B, represent the phase voltages, and R Yand B represent the transformer output voltages.

When S, (FIG. 1) is closed at time x on FIG. 3, the primary of atransformer T, is connected between Y phase and neutral. It will be seenfrom FIG. 2 that during the positive half cycle of the Y phase, both Rand B phases pass through zero voltage, R at rim and negative going, Bat %1r and positive going. As the voltages induced in the secondaries ofT, are negative, and the R phase cross-over is negative going duringthis first half cycle of the Y phase, the secondaries of T, are used tofire the R phase S.C.R.s (i.e. phase retard firing).

As the voltage of the Y phase rises from 0 a D.C. voltage is applied toS.C.R., gate from the secondary of T, (A.B.) via a rectifier D, and thecurrent limiting resistor R,. However, even though the gate voltage ofS.C.R., quickly rises to the firing potential, S.C.R., will not conductbefore the R phase reaches 0, as it is reverse biased. As soon as the Rphase passes through 0 at an Mr, S.C.R., becomes forward biased, and asthere is still a D.C. potential applied to its gate, S.C.R., will nowconduct allowing power to flow in a load L, and a transformer primary TThe voltage of transformer T primary rises negatively and feeds avoltage to S.C.R. gate from T secondary (C.D.) via D and R bringingS.C.R., into condition to conduct as soon as the B phase passes through0 on its positive going sweep at wt %1r. This allows power to flow in aload L and the primary of a transformer T The positive voltage build upin T is fed to S.C.R.,, gate from the transformer secondary (E.F.) via Dand R bringing S.C.R.,, into condition to conduct when Y phase passesthrough 0 on its negative slopeat tot 11', thereby allowing power toflow in a load L At on 71 the Y phase is at 0, therefore, the voltage atthe gate of S.C.R., is 0, and as the Y phase goes negative the voltageinduced in T, secondary (A.B.) is blocked by D,. However, S.C.R.,continues to conduct until the R phase voltage applied to it drops to 0at on (4/3) 1r.

After on 1r, the voltage applied to T, is negative going, and as thisrises a D.C. voltage is applied to S.C.R.,, from T, secondary (G.H.) viaD and R.,, which brings S.C.R., into condition to conduct as soon as theR phase passes 0, out (4/3) 1r, on its positive slope. Power is nowpassed through S.C.R., to load L, and transformer T The voltage of T nowrises positively, supplying a D.C. voltage to the gate of S.C.R. from Tsecondary (I.J.) via D and R permitting S.C.R., to fire at on (5/3) 1r,allowing the B phase to supply power to load L, and transformer T At mt(4/3) 1r, the voltage to the gate of S.C.R., is

blocked by D permitting S.C.R.,, to turn off when the B phase drops to 0at an (5/3) n.

As the B phase rises negatively a D.C. voltage is applied to the gate ofS.C.R.,, from T secondary (K.L.) via D and R bringing S.C.R.,, intocondition to conduct as soon as the Y phase passes through 0 on itspos-' itive slope at (M 2 w.

The voltage to the gate of S.C.R.,, was blocked by D; at on (5/3) 11',so permitting S.C.R. to switch off when the Y phase voltage drops to 0at an 2 1r.

This series of events continues, switching the appropriate S.C.R.s onand off at the zero voltage condition and maintaining full power to thethree-phase load.

For the circuit to operate correctly it is essential that thetransformer phasing is correct.

The description given was for retard firing but by reversal of thetransformer primary connections phase advance firing would result.

The switch-off sequence is as follows:

When the switch 5, is opened at time ,y on FIG. 3, the voltage to thegate of S.C.R. and S.C.R. is removed, so that when the R phase passesthrough 0 the power to load L, and transformer T, is blocked. Thisremoves the gate voltage from S.C.R.,, and S.C.R., so that when the Bphase passes through 0, power is blocked from L and T removing the gatevoltage from S.C.R., and S.C.R.,. As the Y phase subsequently passesthrough 0, power is load from laod L completing the shut down in onehalf cycle.

A practical application of the invention, by way of example only, willnow be described.

-In a blade bonding process where different temperatures are required atdifferent segments of a blade bonding rig, generally indicated at 1, andthese temperatures are required to be kept constant, the presentinvention proved very successful.

The blade bonding machine 1 is divided into a number of sections andeach section into two segments 2 and 3. Each segment is fitted with aplurality of heaters 4, and each segment is supplied separately from thethree-phase mains supply 5, using the present invention 6 forcontrolling the power in each segment.

In each section one segment is controlled from a Programme Controller 7and the other segment from a Deviation Controller 8, the Programme andDeviation Controllers being substituted for switch S of the presentinvention, so that each set of heaters is supplied separately.

A thermocouple feedback 9 to the Programme Controller controls thetemperature of the heaters supplied by the Programme Controller, and aparallel output from the Programme Controller circuit thermocouple,connected, in back to back relationship, with a thermocouple 10 in theheaters supplied by the Deviation Controller, ensures that the DeviationController switches that heater supply when a predetermined temperaturedifference is attained.

Duplicate circuitry is used in each section throughout the length of theblade bonding machine, though the operating temperatures, Programme andDeviation Controllers are set up as required for that particularsection.

With reference to FIG. for the operation of a single phase circuit thevoltage of the secondary windings Sec. 1 and Sec. 2 of the transformer Tshould be as high as possible, to ensure that the S.C.R.s fire earlyenough in each half cycle to be switched when only a very small currentcan flow in the load L, thus .avoiding R.F. radiation due to highcurrent switching.

In single phase circuits it is not possible to arrange the switching ofthe supply at zero volts, i.e. cross-over point, with this type offiring circuit. The actual firing angle may vary from near zero to 90,therefore, care must be exercised in the choice of secondary voltages oftransformer T,. This voltage must be high enough to fire the S.C.R.s yetlow enough to avoid breakdown of the S.C.R. gate from switch-on surges.

If the transformer secondary peak voltage is so low that the gatevoltage just reaches the S.C.R. firing potential, firing of the S.C.R.'swill occur at 90 as this is the angle at which the input and hence thegate voltage, reaches maximum. I

Though it is possible to have a very high voltage fro the secondaries oftransformer T,, and a correspondingly high series resistance to limitthe S.C.R. gate current, thereby achieving a low firing angle,- it wouldbe more advantageous to limit the peak voltage of the secondaries oftransformer T to the maximum gate voltage of the S.C.R. in use, and thenchoose a suitable value of series resistance for correct currentlimiting. Setting the upper voltage in this way will avoid breakdown ofthe S.C.R. gate due to excess voltage being applied if the switch S.W.is closed when the supply waveform is at or near a peak.

By way of am example we will consider the S.C.R. by MullardBTY 34 400Rin the circuit shown (FIG. 5).

Maximum gate voltage 10 V. peak Minimum gate voltage required to triggerall units 3 V. Maximum gate dissipation 5 W. peak' Maximum gate current2 A. peak Minimum gate current 30 mA. peak Maximum forward current 10 A.R.M.S. Therefore, '1 Primary 240 V. R.M.S. Therefore, T Secondaries 7.07V. R.M.S. peak Gate circuit resistance Resistance of secondary forwardresistance of diode R Therefore, Total gate circuit resistance R takingextremities 1009 to 3.39 With R 100.0 the maximum firing angle Sin*(3/10) 17.5 Therefore, Supply voltage at l7.5 340 X 0.30071 102 V. Atfull load 14.14A. peak load voltage 340 V. peak Therefore, Z,,(340/1414) 240. Therefore, In the worst case load current at firingangle l7.5 (102/24) 4.25A. i.e. 30 percent of full load To bring thefiring angle down to a more acceptable limit a transformer secondaryvoltage of a higher value may be used, e.g. 25 V. peak.

With R 1000 the maximum firing angle Sin "(3/25) z 7 Supply voltage at 7340 X 0.12 41 V.

Therefore, In the worst case load current at firing angle 7 (41/24) 1.7i.e. 12 percent of full load When reducing the firing angle in this wayit is advisable to include some sort of protection for the S.C.R. Thismay take the form of a zener diode across the gate circuit as shown inFIG. 5 by the broken lines (D and D Various amendments to the circuitswill become apparent without deviating from the invention, e.g. theinvention could be used for delta connected loads, triacs could be usedin place of S.C.R.s, in which case each transformer would require onlyone secondary and the diodes would be dispensed with, or by carefuldesign and manufacture of the transformer the diodes and resistors couldbe dispensed with. 8,, though shown as a mechanical device, couldequally well be a semiconductor switch controlled by any electronicdevice such as a cross-over point detector, burst firing module, or anadjustable on-off ratio controller. Half power can be supplied by theinsertion of a switch in lieu of the resistor in one of the twosecondaries in each transformer, permitting the same sense secondariesto be open circuited. The circuit, shown controlling a resistive load,could equally control a capacitive, inductive or the like load, andcould also be utilised for the speed control of motors, therebyconserving power.

We claim as our invention:

1. A control circuit for switching single and polyphase supplies, saidcontrol circuit comprising thyristor means for controlling connection ofelectrical load meansto the electrical supply, firing means forcontrolling triggering of said thyristor means such that the electricalload means is connected to the electrical supply at or near zero volts,and a single pole switching means located within a single phase of thesupply for controlling energization of said firing means.

2. A control circuit for switching single phase electrical supplies,said control circuit comprising thyristor means for controlling theconnection of electrical load means to a single phase electrical supply,firing means for controlling triggering of said thyristor means suchthat the electrical load means is connected to said single phase supplywhen the single phase supply voltage is near zero volts, and a singlepole switch connected in series with said firing means across saidsingle phase for controlling energization of said firing means.

3. A control circuit for switching polyphase electrical supplies, saidcontrol circuit comprising thyristor means located in each phase of apolyphase supply for controlling the connection of electrical load meansto the supply, firing means associated with each phase for respectivelytriggering corresponding ones of said thyristor means, a single poleswitching means for, when actuated, causing energization of a first saidfiring means, and means for connecting said thyristor means and saidfiring means such that energization of said first firing means causestriggering of a said thyristor means in the succeeding phase andtriggering of that thyristor means causes energization of further firingmeans in the same phase and such that the firing means of each of theother phases triggers the thyristor means in each succeeding phasewhereby each phase sequentially switches the succeeding phase, saidsequential switching occurring as each phase passes through zero voltsin both positive and negative senses.

4. A control circuit for switching polyphase electrical supplies, saidcontrol circuit comprising thyristor means located in each phase of apolyphase supply for controlling the connection of electrical load meansto the supply, firing means associated with each phase for respectivelytriggering corresponding ones of said thyristor means, a single poleswitching means for, when actuated, causing energization of a first saidfiring means, and means for connecting said thyristor means and saidfiring means such that energization of said first firing means causestriggering of a said thyristor means in the preceding phase andtriggering of that thyristor means causes energization of further firingmeans in the same phase and such that the firing means of each of theother phases triggers the thyristor means in each preceding phasewhereby each phase sequentially switches the preceding phase, saidsequential switching occurring as each phase passes through zero voltsin both positive and negative senses.

5. A control circuit as claimed in claim 1, wherein said firing meanscomprise transformer means having two independent, rectified and currentlimited outputs 9. A control circuit as claimed in claim 1, wherein saidsingle pole switching means comprises a semiconductor controlled by across-over point detector.

10. A control circuit as claimed in claim 1, wherein said single poleswitching means comprises a semiconductor controlled by a burst firingmodule.

11. A control circuit as claimed in claim 1, wherein said single poleswitching means is a semi-conductor controlled by an adjustable ON/OFFratio controller.

12. A control circuit as claimed in claim 1 for use in the speed controlof motors.

13. A control circuit as claimed in claim 2, wherein a voltage controldevice is provided to protect said thyristor means.

14. A control circuit as claimed in claim 1, wherein further switchingmeans are provided for isolating part of said firing means for supplyinghalf power.

15. A control circuit as claimed in claim 1, for use in controllingpower supplied to a blade bonding machine.

1. A control circuit for switching single and polyphase supplies, saidcontrol circuit comprising thyristor means for controlling connection ofelectrical load means to the electrical supply, firing means forcontrolling triggering of said thyristor means such that the electricalload means is connected to the electrical supply at or neaR zero volts,and a single pole switching means located within a single phase of thesupply for controlling energization of said firing means.
 2. A controlcircuit for switching single phase electrical supplies, said controlcircuit comprising thyristor means for controlling the connection ofelectrical load means to a single phase electrical supply, firing meansfor controlling triggering of said thyristor means such that theelectrical load means is connected to said single phase supply when thesingle phase supply voltage is near zero volts, and a single pole switchconnected in series with said firing means across said single phase forcontrolling energization of said firing means.
 3. A control circuit forswitching polyphase electrical supplies, said control circuit comprisingthyristor means located in each phase of a polyphase supply forcontrolling the connection of electrical load means to the supply,firing means associated with each phase for respectively triggeringcorresponding ones of said thyristor means, a single pole switchingmeans for, when actuated, causing energization of a first said firingmeans, and means for connecting said thyristor means and said firingmeans such that energization of said first firing means causestriggering of a said thyristor means in the succeeding phase andtriggering of that thyristor means causes energization of further firingmeans in the same phase and such that the firing means of each of theother phases triggers the thyristor means in each succeeding phasewhereby each phase sequentially switches the succeeding phase, saidsequential switching occurring as each phase passes through zero voltsin both positive and negative senses.
 4. A control circuit for switchingpolyphase electrical supplies, said control circuit comprising thyristormeans located in each phase of a polyphase supply for controlling theconnection of electrical load means to the supply, firing meansassociated with each phase for respectively triggering correspondingones of said thyristor means, a single pole switching means for, whenactuated, causing energization of a first said firing means, and meansfor connecting said thyristor means and said firing means such thatenergization of said first firing means causes triggering of a saidthyristor means in the preceding phase and triggering of that thyristormeans causes energization of further firing means in the same phase andsuch that the firing means of each of the other phases triggers thethyristor means in each preceding phase whereby each phase sequentiallyswitches the preceding phase, said sequential switching occurring aseach phase passes through zero volts in both positive and negativesenses.
 5. A control circuit as claimed in claim 1, wherein said firingmeans comprise transformer means having two independent, rectified andcurrent limited outputs of opposed polarity.
 6. A control circuit asclaimed in claim 1, wherein said thyristor means comprise a plurality ofsilicon controlled rectifiers connected within each phase to switchpositive and negative half cycles respectively.
 7. A control circuit asclaimed in claim 1, wherein said firing means comprises a transformerwith a single secondary winding and said thyristor means comprisetriacs.
 8. A control circuit as claimed in claim 1, wherein said firingmeans comprise a transformer providing non-rectified, non-currentlimited control of said thyristor means.
 9. A control circuit as claimedin claim 1, wherein said single pole switching means comprises asemi-conductor controlled by a cross-over point detector.
 10. A controlcircuit as claimed in claim 1, wherein said single pole switching meanscomprises a semi-conductor controlled by a burst firing module.
 11. Acontrol circuit as claimed in claim 1, wherein said single poleswitching means is a semi-conductor controlled by an adjustable ON/OFFratio controller.
 12. A control circuit as claimed in claim 1 for use inthe speed control of mOtors.
 13. A control circuit as claimed in claim2, wherein a voltage control device is provided to protect saidthyristor means.
 14. A control circuit as claimed in claim 1, whereinfurther switching means are provided for isolating part of said firingmeans for supplying half power.
 15. A control circuit as claimed inclaim 1, for use in controlling power supplied to a blade bondingmachine.